In particular, wind turbines (WTs) are systems which are susceptible to vibration. In order to keep the vibration amplitude of the WT low in the case of resonance stimulation, damping devices are usually integrated into the WT system. These dampers must be matched to the resonance frequency of the respective WT. The stiffness of the WT, its mass and the stiffness of the pedestal, which also depends on the characteristics of the ground, are primarily determined here by the inherent frequency of the WT.
A stiff pedestal results in a high resonance frequency of the WT and vice versa. A lower mass results in a higher inherent frequency of the WT and vice versa. During installation of the structure, precisely this situation applies. There are temporary states in which only the tower without nacelle and rotor can be stimulated to resonance by the wind. For this case, the inherent frequency of the semifinished WT is much higher than in the case of the later fully installed structure. A similar situation may occur during erection or construction of tall slim edifices (towers, multistorey buildings, etc.).
In order to be able to obtain optimum damping performance, the damper frequency must be adaptable to the respective structure frequency, particular tower frequency, and variable in a large range.
If, for example, a simple pendulum damper (variant 1; FIG. 1) is regarded as a point mass, its frequency is only dependent on the pendulum length l1 (2). The physical correlation can be described as follows:
  f  =            1              2        *        π              *                  g                  l          ⁢                                          ⁢          1                    
g→gravitation constant [m/s^2]
l1→pendulum length [m]
The frequency can thus only be influenced by the pendulum length. In practice, in particular in space-restricted WTs, this can by contrast, only be modified to a limited extent.
In order to obtain further adjustment possibilities for the damper frequency of a pendulum damper, variant 1 can usually be supplemented with horizontal springs. In the case of variant 2.1, these horizontal springs (4) act at the height of the damper mass (3). In the case of variant 2.2, these are moved in the direction of the suspension point (1) (FIG. 2).
This physical correlation of systems 2.1 and 2.2 can be described as follows:
            f      =                                    1                          2              *              π                                *                                                                      C                  *                                                            l                      2                                        ^                    2                                                                    m                  *                                                            l                      s                                        ^                    2                                                              +                              g                                  l                  1                                                                    →                  Equation          ⁢                                          ⁢          1                      ;          Variant      ⁢                          ⁢      2.2                  f      =                                    1                          2              *              π                                *                                                    C                m                            +                              g                                  l                  1                                                                    →                  Equation          ⁢                                          ⁢          2                      ;          Variant      ⁢                          ⁢      2.1      
g→gravitation constant [m/s^2]
l1→pendulum length [m]
l2→length from pivot to point of action of horizontal spring [m]
C→spring stiffness of horizontal spring [N/m]
m→damper mass [kg]
it can be seen that the damper frequency can now be adjusted via a number of parameters.
Such solutions have already been described frequently in the prior art. The horizontal springs (4) are subjected to tensile stress. Due to the position of installation, the horizontal springs (4), in the case of variant 2.1, see the same displacement as the damper mass (3). They are consequently subjected to very high dynamic loads, which makes a durable design, for the given construction space, very difficult to impossible.
In order to minimise the displacement of the horizontal springs (4), they can be moved further in the direction of the suspension point of the damper (1) (variant 2.2). In this position, they see less displacement, but in this constellation the spring stiffness of these springs must increase in order to achieve the same performance compared with variant 2.1. In addition, this requires a flexurally stiff pendulum length (2), which more or less excludes the use of cables.
If the mass moves to the right, the left-hand horizontal spring (4) is stretched further and the right-hand horizontal spring (4) is relieved of load. In this constellation, it must be ensured that the relieving spring is not completely relieved of load. This means that this horizontal spring (4) is pre-tensioned more in the middle position than the vibration displacement of the damper mass (3) can be. This fact makes installation difficult.
As already mentioned, the damper frequency is purely dependent on the spring stiffness of the horizontal springs (4) and the pendulum length. If it is now intended to set a different frequency, it is necessary to install a horizontal spring (4) having a different spring stiffness for a given, constant pendulum length (2), as is likewise frequently found in the prior art. By contrast, an increase in the spring pretension force has no influence on the damper frequency.